Cathode ray tubes (CRTs) are one of generally used display devices and have been mainly used in monitors of measuring instruments, information terminals, as well as in televisions. However, the CRTs are heavy and bulky and thus, do not cope with requirements for smaller size and light weight electronic products.
Accordingly, the CRTs have limits in size, weight, and other characteristics and are problematic to follow the tendency of the ongoing of the size and weight of a variety of electronic products. To substitute for the CRTs, there have been developed liquid crystal display (LCD) devices using electric-field optical effect, plasma display panels (PDPs) using gas discharge, electroluminescence display (ELD) devices using electric-field emission effect, and the like.
Liquid crystal display devices have advantages of small size, light weight, and low consumption of electric power, etc. required to substitute for the CRTs. Recently, the liquid crystal display devices have been developed to perform the role of flat panel display devices, and used in laptop computers as well as monitors of desktop computers, and other large-scale information display devices. The demand for liquid crystal display devices is increasing continuously.
Such a liquid crystal display device may be basically divided into a liquid crystal panel for displaying images thereon and a drive unit for applying a driving signal to the liquid crystal panel. The liquid crystal panel includes first and second glass substrates bonded to each other with a predetermined space therebetween, and a liquid crystal layer injected between the first and second glass substrates.
Specifically, the first glass substrate (TFT array substrate) is formed with a plurality of gate lines arranged in a direction to be spaced apart from each other by a predetermined distance, a plurality of data lines arranged orthogonal to the respective gate lines to be spaced apart from each other by a predetermined distance, a plurality of pixel electrodes formed at respective pixel regions where the gate lines and data lines intersect with each other, the pixel electrodes defining a matrix, and a plurality of thin-film transistors adapted to be switched by signals from the gate lines, so as to transmit signals from the data lines to the respective pixel electrodes.
The second glass substrate (color filter substrate) is formed with a black matrix layer for shielding light at a portion except for the pixel regions, red (R), green (G), and blue (B) color filter layers for representing a variety of colors, and a common electrode for realizing images.
The first and second glass substrates are bonded to each other by means of a sealing material pattern having a liquid crystal injection port in such a manner that they have a predetermined separation through the use of a spacer. Liquid crystals are injected into the space between the substrates.
Most liquid crystal display devices are light-receiving devices and adapted to display images by regulating the amount of light incident from the outside passing therethrough. Therefore, the liquid crystal display devices need a separate light source for irradiating light to a liquid crystal panel, such as a backlight unit. The backlight unit as a separate light source is may be an edge type backlight unit or a direct type backlight unit, based on the installation positions of lamp units.
Examples of the light source for the liquid crystal display devices include electro luminescence (EL), light emitting diode (LED), cold cathode fluorescent lamp (CCFL), hot cathode fluorescent lamp (HCFL), external electrode fluorescent lamp (EEFL), and the like.
The CCFL, HCFL, and EFFL are linear light sources, and an LED is a point light source.
The backlight unit has been used to assist a user to read information displayed on a screen of a liquid crystal display device in a dark place, and in response to a variety of requirements, low consumption of electric power, overall thinness, a reduced thickness light guide plate of the backlight unit and the like. Backlight units are being developed to have a function of representing a variety of colors and to achieve a reduction in the consumption of electric power through the use of LEDs.
A light emitting diode (LED) is a solid state device using photoelectric transformation effect of semiconductors, and adapted to emit light when a forward voltage is applied thereto. The LED is able to emit light at a lower voltage than tungsten filament lamps and, more particularly, to emit light based on a difference of energy that is generated during an electron-hole recombination, rather than emitting light via heating of filaments. Therefore, the LED has been widely used in a variety of display devices.
Using the LED as a light source for illuminating a liquid crystal panel is efficient to provide electronic appliances, such as laptop computers, etc., with outstanding characteristics, such as smaller size and low consumption of electric power.
Since the LED can emit light if a low voltage DC power is applied thereto, it has no necessity for a DA-AC converter. Consequently, the LED has a feature of a simplified driving unit, thus achieving a considerable reduction in the consumption of electric power.
Also, the LED is a semiconductor device and therefore, has a higher reliability, smaller size, and longer lifespan than cathode ray tubes.
A conventional backlight unit, as shown in FIGS. 1 and 2, includes a bottom cover 10, a printed circuit board (PCB) 11 attached to an inner upper surface of the bottom cover, a plurality of LED lamps 12 successively mounted on an upper surface of the printed circuit board 11 to extend in a direction, and a reflecting plate 13 disposed over the printed circuit board 11 and adapted to transmit light generated from the LED lamps 12 upward.
Side supporters 14 are provided at opposite sides of the printed circuit board 11, to support the printed circuit board 11. A light diffusion unit 15 is disposed over the arranged LED lamps 12. In this case, the light diffusion unit 15 includes a diffusion plate and a plurality of optical sheets.
Each of the LED lamps 12, as shown in FIGS. 3A and 3B, includes a combination of red (R), green (G), and blue (B) LED chips 17a, 17b, and 17c. Since all the LED lamps 12 have the same configuration as each other, the following description deals with only one LED lamp 12. In the shown conventional example, more particularly, two green LED chips 17b are arranged diagonally to each other, and one red LED chip 17a and one blue LED chip 17c are arranged diagonally to each other so that each is on an opposite side of its horizontally adjacent green LED chip 17b compared to the other.
The LED lamp 12 is divided into a lower body portion 12a and an upper light emitting portion 12b. The reflecting plate 13 has a plurality of holes 16 formed at positions thereof corresponding to the light emitting portions 12b of the LED lamps 12, respectively, such that the reflecting plate 13 is fitted at a boundary region between the body portion 12a and the light emitting portion 12b of the respective LED lamps 12. In this case, the light emitting portions 12b of the LED lamps 12 have dome-shaped optical lenses.
The holes 16 are vertically formed in the reflecting plate by a simple punching process.
If light is emitted from the LED lamp 12 including the red, green, and blue chips 17a, 17b, and 17c, the light is reflected by the light diffusion unit 15 or side supporters 14. The reflecting plate 13 having the above described configuration serves to again reflect the light, so as to emit the reflected light to the outside from an upper surface of the backlight unit.
However, the respective red, green, and blue LED chips 17a, 17b, and 17c included in the LED lamp 12 are arranged in such a manner that they are slightly deflected from the center of the LED lamp 12. With this deflected chip arrangement, the light emitting portion, i.e. optical lens, emits a slightly greater amount of light in a direction close to an edge thereof. Also, a laterally emitted part of the light emitted from the light emitting portion 12b formed by the optical lens collides with a vertical section of each hole 16 formed in the reflecting plate 13, thereby being bent toward a diagonal edge portion and reflected upward.
In this case, since the holes 16 are formed vertically in the reflecting plate 13 by a simple punching process, as shown in FIGS. 4A, 4B, and 5, the light transmitted to the vertical section of each hole 16 is bent and reflected in a direction opposite to the associated LED chip. As a result, the LED lamp 12 shows color separation phenomenon in that a diagonal portion of the LED lamp 12 where the red LED chip 17a is located represents a reddish color, and an opposite diagonal portion where the blue LED chip 17c is located represents a bluish color.
Since the holes 16, which are used to expose the light emitting portions 12b of the LED lamps 12 to the outside, are punched vertically in the reflecting plate 13, the light emitted from the respective LED chips is directed in a diagonal direction after colliding with the vertical section of each hole 16. This causes a color separation phenomenon between opposite diagonal portions of the respective LED chips.